1. Field of the Invention
The present invention relates to a survey meter for inspecting whether an object is radioactively contaminated or not by measuring an amount of radiation irradiated from the object, and more particularly, relates to a survey meter having a structure capable of directly measuring the radioactive contamination caused on inner surface of a pipe, tube or the like having a small diameter.
2. Related Art
In a radiation handling facility such as a nuclear power station or the like, a law and relating regulations stipulate and oblige the facility to set a radiation management (controlled) area or site. Further, the law also obliges all objects and substances to undergo careful inspection for the radioactive contamination before the objects or the like are transported to outside from the radiation management area or site.
As a main inspection apparatus for inspecting whether the radioactive contamination is caused or not, there has been used a surface contamination survey meter. The surface contamination survey meter is classified and controlled so as to be selectively used in accordance with a kind and intensity of the radioactive rays, so that an operation for measuring the intensity of the radioactive rays using the survey meter can be normally performed under optimum conditions.
A representative example of a structure of the surface contamination survey meter is shown in FIG. 5. The survey meter includes: a scintillator unit 2 for emitting a fluorescent light generated by the radioactive ray irradiated from a surface of a measuring object (object to be measured) 16; a light guide unit 1 for converging the fluorescent light; a photoelectric transfer unit 3 for converting the fluorescent light into an electric signal; a pulse height discriminator 7 for discriminating the electric signal from a noise by a threshold value predetermined from a pulse height of the electric signal; a counter unit 8 for counting a signal outputted from the pulse height discriminator 7; and a display unit 10 for displaying a signal counting rate.
The above structures have been disclosed in the following patent documents. Namely, Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2005-241447) discloses a radioactivity measuring apparatus using an optical fiber. Further, Patent Document 2 (Japanese Unexamined Patent Application Publication No. 2005-265498) discloses a surface contamination measuring apparatus and a radioactivity detecting unit.
In the above conventional surface contamination survey meters, the radiation detecting units are formed to provide various shapes such as rod-shape, conical-shape, flat plate-shape or the like. However, the radiation detecting units are basically designed on the assumption that the radiation detecting units are normally used for inspecting the radioactive contamination caused on an outer surface of the object to be measured. Therefore, with respect to a large contamination generating amount as waste of the pipe, tube or the like each having a small diameter, there has been posed a problem to directly measure an inner surface contamination of the pipe or tube in view of physical configuration and size of the radiation detecting units.
Further, for example, the above conventional “radioactivity measuring apparatus using an optical fiber” disclosed in the Patent Document 1 is configured such that a wavelength conversion optical fiber is provided so as to connect a portion between a long plate-shaped radiation detecting unit having a scintillator and a radiation measuring unit provided with a photo-multiplier tube, and a signal caused by α-rays detected by the radiation detecting unit is transmitted to the radiation measuring unit through the optical fiber. The Patent Document 1 states that the radioactivity measuring apparatus can measure the contamination caused on the inner surface of the pipe or the like.
However, in the conventional survey meter having the above structure, the radiation detecting unit is formed to provide a flat plate-shape. Therefore, in case of measuring the inner surface of the tube or the like, a distance between the inner surface of the pipe and a detecting surface is unavoidably apart to thereby form a disadvantageous gap therebetween. As a result, a radiation is attenuated and lowers a detecting efficiency. In particular, when the radioactive ray is α-ray or β-ray of a low energy level, the lowering of the detecting efficiency becomes more remarkable in comparison with a case where the surface of the measuring object and the detecting surface are tightly contacted to each other.
Further, the wavelength conversion optical fiber has a relatively large light transmission loss due to self-absorption in comparison with a transparent and colorless optical fiber. This light transmission loss is one factor of lowering the detecting efficiency. Accordingly, there has been posed a problem that it is impossible to secure a sufficient distance between the radiation detecting unit and the radiation measuring unit.
Furthermore, the above conventional “surface contamination measuring apparatus and a radioactivity detecting unit” disclosed in the Patent Document 2 is configured such that a scintillator is attached to an outer peripheral surface of a slender cylindrical pipe capable of transmitting a light, and one or more of the photo multiplier tubes are fixed to the pipe in a lance manner. In the latter case of fixing the plurality of photo-multiplier tubes, since output signals from adjacent photo multiplier tubes are added and revised so as to effectively suppress the deterioration of the detecting accuracy.
However, in the conventional survey meter having such structures as mentioned above, a size of the photo-multiplier tube is so large to occupy a large installation volume, so that there has been posed a problem and restriction that a diameter of the detecting portion cannot be formed to be smaller than that of the photo-multiplier tube. In addition, when the detecting portion is formed to be long, it becomes necessary to provide a plurality of photo-multiplier tubes, resulting in a high cost involved for manufacturing the survey meter. Further, since a gross weight of the survey meter is also increased, it becomes difficult and not easy to handle the survey meter.
As mentioned above, the detecting portion having the conventional structure cannot directly measure the radioactive contamination caused in a pipe or tube having a small diameter, the measuring operation cannot be performed with a high efficiency, the stabilization of a detecting sensitivity is difficult when the detecting portion is formed to be long, a manufacturing cost including working cost is disadvantageously high and the handling of the survey meter is not easy at the time of measuring operation.